Extra vertex in Cycles render
What you have is an extra edge. From what can be told from the picture, the positions of the vertices forming that face are not flat, and so you are asking it to make an impossible shape. You cannot have a square with diagonal verts at different heights that do not create a flat plane. It is showing the triangle because you are asking for something that is not possible.

Smooth shading may help. But for the shape you want, you need more geometry for it to be possible.